Bone screws and wires find wide-ranging applications in osteosynthesis. In osteosynthesis bone screws are used for fixation of bone fragments as well as attachment of longitudinal support rods. Wires are used to fix bone fragments.
Conventional bone screws, particularly the so-called Schanz screws, are used in osteosynthesis for a multiplicity of purposes in positioning and fixing bone fragments. An example of this are components of an osteosynthetic external fixation device (such as that of EP-B1 0 153 546), which in essence consists of one or more connected longitudinal support rods, that can be disconnected from each other, along with the attached clamps or clasps for receiving the Schanz screws. Such an application requires that prior to the placement of the Schanz screws an appropriate hole be drilled into the bone, into which the Schanz screw can then be screwed. This procedure is complex and time-consuming.
The thread pitch of a bone screw is critical, for it determines the cutting rate (i.e., the advance) of the screw. In other words, for each turn the screw is forced to go forward by the length of the screw pitch. If a screw pitch of about 1.75 mm, typical for bone screws, is selected, then the tip of the tap used to thread the hole will advance an increment of 1.75 mm for each turn. Assuming a drilling machine with a normal rotational speed of about 600 to 800 r.p.m., typically employed in bone surgery is used to drive the tap, this results in an advance of more than a meter per minute. The extraordinarily intense pressure on the bone caused by this advance to the tap tip can result in spontaneous tears or fractures in the bone. The principal disadvantage of having too rapid an advance is evident primarily when the tip has passed through the proximal corticalis and the medullary space and encounters the inner side of the counter-corticalis. There the tip has no chance of centering itself and producing a so-called channel. Instead of the bone being drilled through, it is thrust away by the tip. This leads inevitably to a splitting of the counter-corticalis. If the screw is inserted near the fracture, the applied pressure is dispersed through longitudinal fissures of the counter-corticalis.
Efforts to reduce the axial pressure by reducing the drill r.p.m. are not possible, since the advance is preset by the thread pitch of the screw.
The manipulation procedures for the insertion of bone screws are also extended. The screw hole must first be bored out with an initial instrument, and then a second instrument must be used to cut the thread. Finally, in a third procedure, the bone screw is screwed in. It is also not uncommon to have a typical bone screw tightened to the limit of its retention force, in order to fix longitudinal support rods or bone plates on the bone. This is necessary since there is no stable connection between bone screws and the support rods or plates. Owing to the often very high initial stress, up to the retention force limit, screws can tear loose from the bone.
In addition, during the procedure described above, errors can accumulate, resulting in a poor fixation.
Wires can be inserted into bone without large-scale preliminary work, but they damage the bone owing to the associated high temperatures which arise from friction between bone and wire. In addition, the axial stability of bone fragments fixed with wires is not very great, so that the result may be a loss of positioning.